1. Field of the Invention
The present invention relates to an antenna for a radio telephone, and more particularly to an antenna for a radio telephone for use in a cellular, PHS (Personal Handy Phone System), or other mobile phone system.
2. Description of the Related Art
Today, cellular, PHS, and other mobile phone systems enjoy widespread use. The radio telephones (telephones) used in such systems have in recent years become increasingly more compact and lightweight, creating a need to make the antennas used therefor smaller in association with the smaller size and lighter weight of the radio telephones.
An extendible/retractable antenna used by extending a rod-shaped antenna from the case of the radio telephone to communicate in locations where electric field strength is weak, and retracting the rod-shaped antenna into the case of the radio telephone to make it convenient to hold and walk when being carried about, and that stands by to receive calls with a helical antenna alone is commonly known as a conventional radio telephone antenna of this type.
For example, there is known an antenna of the kind depicted in FIG. 11, wherein a conductor 71 constituting the main body of a rod-shaped antenna 70 is provided at the distal end thereof via an insulating connecting member 72 and a conducting connector 82 with a helical coil 81 constituting the main body of a helical antenna 80 (hereinafter termed xe2x80x9cantenna of Conventional Art Example 1xe2x80x9d). Where xcex represents the wavelength of the service frequency, the electrical length of the rod-shaped antenna 70 is established in the vicinity of xcex/4 and the electrical length of the helical antenna 80 is established at xcex/4. When the antenna of Conventional Art Example 1 is extended from the case 50 of the radio telephone (FIG. 11a), a high-frequency electric current from a power supply point 41 provided to the circuit board 40 of the radio telephone is supplied through a connecting lead 42 and via a metal retaining member 91 and a fastener 92 so as to excite the antenna 70. The helical coil 81 provided to the distal end of the conductor 71 is insulated by means of the connecting member 72, so the high-frequency electric current supplied by the power supply point 41 is obstructed by the connecting member 72 and does not excite the helical antenna 80. That is, in the state of the antenna being extended from the case 50, radio waves are emitted from the rod-shaped antenna 70 exclusively, and radio waves are not emitted from the helical antenna 80.
Meanwhile, when the rod-shaped antenna 70 is retracted within the case 50 (FIG. 11b), high-frequency electric current supplied by the power supply point 41 of the radio telephone through the connecting lead 42 and via the conducting connector 82 excites the helical antenna 80. The rod-shaped antenna 70 retracted within the case 50 is not excited since the high-frequency electric current is obstructed by the connecting member 72.
That is, as shown in FIG. 12 the antenna of Conventional Art Example 1 is such that when the rod-shaped antenna 70 is extended from the case 50 for use, sending and reception are carried out exclusively by the rod-shaped antenna 70 of electrical length in the proximity of xcex/4, and where the rod-shaped antenna 70 is retracted within the case 50 for use, sending and reception are carried out exclusively by the helical antenna 80 of electrical length of xcex/4.
Also known is an antenna like that depicted in FIG. 12, wherein a helical coil 81 is arranged at the distal end of a conductor 11 via a conducting connector 73 only (hereinafter termed xe2x80x9cantenna of Conventional Art Example 2xe2x80x9d). A rod-shaped antenna 74 comprises a conductor 71 and a connector 73, the electrical length thereof being established at xcex/2, including the conductor 71 and the connector 73. The electrical length of a helical antenna 80 comprising a helical coil 81 is established at xcex/4.
The conductor 71 and the helical coil 81 are electrically connected by means of the connector 73, and thus when the antenna of Conventional Art Example 2 is in a state extended from the case 50 (FIG. 12a), high-frequency electric current from a power supply point 41 supplied through a connecting lead 42 and via a metal retaining member 91 and a fastener 92 excites the rod-shaped antenna 74 as well as exciting the helical antenna 80. The electrical length of the rod-shaped antenna 74 is xcex/2 and the electrical length of the helical antenna 80 is xcex/4, so in a state with the rod-shaped antenna 74 extended from the case 50, the total electrical length of the rod-shaped antenna 74 and the helical antenna 80 is 3xcex/4.
Meanwhile, in a state with this antenna retracted within the case 50 (FIG. 12b), high-frequency electric current from the power supply point 41 of the radio telephone supplied through the connecting lead 42 and via the connector 73 excites not only the helical antenna 80 but also the rod-shaped antenna 74 retracted within the case 50.
That is, this antenna of Conventional Art Example 2 is such that, in the event that the rod-shaped antenna 72 is extended from the case 50, the rod-shaped antenna 3 of electrical length of xcex/2 and the helical antenna 4 of electrical length of xcex/4 are simultaneously excited to carry out sending and reception as an antenna of electrical length of 3xcex/4, and in the event of use with the rod-shaped antenna 74 retracted within the case 50, sending and reception are carried out through parallel operation of the rod-shaped antenna 74 of electrical length of xcex/2 and the helical antenna 80 of electrical length of xcex/4.
As is well known, emission within a vertical plane by a rod-shaped antenna of electrical length of xcex/4 is directed downward with respect to the horizontal direction,. which is a cause of lowered sensitivity in the horizontal direction. Accordingly, the antenna of Conventional Art Example 1 having a rod-shaped antenna 70 of electrical length of xcex/4 has this problem of lowered sensitivity. Since the rod-shaped antenna 70 is a grounded type antenna, when the radio telephone is used by pressing against the ear, there is the problem of the impedance characteristic, etc., fluctuating due to the effects of the human body.
Further, in the antenna of Conventional Art Example 1, in the state with the rod-shaped antenna 70 extended from the case 50, the helical antenna 80 functions merely as a grip for extending and retracting the rod-shaped antenna 70 with respect to the case 50, and efficiency is poor given the size of the antenna.
Since it is common to manufacture the conductor 71 using a member of small diameter having good elasticity, the arrangement of a helical coil 81 having large mass at the distal end thereof creates susceptibility to vibration due to external pressure, the amplitude thereof being large, creating susceptibility to mechanical or electrical noise and posing the risk that the retaining power of the retaining member 91 may deteriorate.
Further, the connecting member 72 of this antenna of Conventional Art Example 1 has a problem in terms of flexural strength, and depending on the length thereof breakage may occur with repeated flexing. Improving the flexural durability requires means such as lengthening the length, etc., but by so doing the total length of the rod-shaped antenna 70 will become physically long, creating the problem of not being retractable into the case at the time of retraction.
Meanwhile, as is well known, emission within a vertical plane by an antenna of electrical length of 3xcex/4 is split into two directions with respect to the horizontal direction, with the main emission being upward-directed, and remains slightly in the horizontal direction as sub-emission. Accordingly, in the antenna of Conventional Art Example 2, wherein transmission and reception are carried out as an antenna of electrical length of 3xcex/4 when the rod-shaped antenna 74 is extended, in the state with the rod-shaped antenna 74 extended from the case 50, there will be as a result the problem of lowered sensitivity in the aforementioned horizontal direction. This antenna of Conventional Art Example 2 is a grounded type antenna as is the antenna of Conventional Art Example 1, and thus when the radio telephone is used by pressing against the ear, there is the problem of the impedance characteristic, etc., fluctuating due to the effects of the human body.
Further, the length of the antenna of Conventional Art Example 2 is close to twice the length of the antenna of Conventional Art Example 1, and, as in Conventional Art Example 1, it is common to manufacture the conductor 11 using a member of small diameter having good elasticity; further, a helical coil 81 having virtually the same weight as in Conventional Art Example 1 is provided at the distal end thereof, creating susceptibility to vibration due to external pressure, the amplitude thereof being large, creating susceptibility to mechanical or electrical noise and posing the risk that the retaining power of the retaining piece may deteriorate. Further, the antenna of Conventional Art Example 2 has the problem that the electrical length of the rod-shaped antenna 74 is long when retracted, so that it cannot be physically accommodated within the increasingly more compact cases of recent years.
Further, when the rod-shaped antenna 74 is retracted within the case 50, not only the helical antenna 80 but also the rod-shaped antenna 74 retracted within the case 50 are excited, with the result that radio waves can be emitted within the case 50 as well. When used in a location with strong reception or electric field strength, for example, unwanted radio waves are emitted from the rod-shaped antenna 74 with the case (hereinafter termed xe2x80x9cunwanted radiationxe2x80x9d), causing interference, etc., and posing the problem of adverse effects on the electric circuits within the case 50. To prevent this, there is required special means such as electromagnetically shielding the portion housing the rod-shaped antenna 74. However, this runs counter to the trend of compactness and lighter weight on the part of radio telephones.
As a way to resolve the problems pertaining to the antennas of Conventional Art Examples 1 and 2, and particularly the problem of electrical length with the rod-shaped antenna extended from the case and of unwanted radiation with it retracted within the case, there has been proposed an antenna wherein the electrical length of the rod-shaped antenna is set to 3xcex/8 and the electrical length of the helical antenna to xcex/4, and designed such that when the rod-shaped antenna is extended to the outside of the case, the basal end of the conductor and the basal end of the helical coil are connected in common to a power supply portion and co-operate, and when the rod-shaped antenna is retracted within the case, only the helical antenna is connected to the power supply portion (see Japanese Unexamined Patent Publication No. 8(1996)-23216; hereinafter xe2x80x9cConventional Art Example 3xe2x80x9d).
However, the antenna of Conventional Art Example 3 has the problem of being impossible to realize in terms of antenna engineering. Specifically, it is stated that, xe2x80x9cwhen the rod-shaped antenna element is extended to the outside of the case, the basal end of the rod-shaped antenna element (conductor) and the basal end of the helical antenna element (helical coil) are connected in common to a power supply portion. Thus, the two antennas co-operatexe2x80x9d. However, the impedance of the helical antenna of electrical length of xcex/4 is nominally Z=Rxc2x1j0, and the impedance of the other rod-shaped antenna of electrical length of 3xcex/8 is Z=R+jX, and thus where the two antennas are connected at the basal ends thereof and the two antennas are postulated to be R=50 ?, the composite impedance is Zr=50/2+jX=25+jX. Further, since the means for canceling out the inductive reactance+jX of the rod-shaped antenna is unclear, base impedance of Z=50xc2x1j0 on the part of the antenna cannot be realized, and thus the antenna is not realizable.
As shown in FIGS. 11 and 12, in the antenna of either Conventional Art Example 1 or 2 the metal retaining member 91 etc. is arranged in proximity to the power supply portion 41, thus creating between the retaining member 91 and the power supply portion 41 a large free electrostatic capacitance that cannot be ignored in terms of antenna engineering, and resulting in impaired impedance matching. Specifically, in light of the wide service frequency band assigned to portable radio telephones these days, adapting to wider bandwidth through impedance matching means that creation of the aforementioned free electrostatic capacitance will make it difficult to adapt to the wider bandwidth. Another problem associated with creation of free electrostatic capacitance is lowered gain and sensitivity.
Further, in the event that the radio telephone should be accidentally dropped on the floor in a state with the rod-shaped antenna retracted within the case 50, a concentrated impact load will be applied to the base of the helical antenna 80, creating the risk that the helical antenna 80 will break. In the event that such an accident should happen, it will be necessary to change and replace the antenna portion. In such an event, it will be necessary to remove the retaining member 91 from the case 50 and replace both the rod-shaped antenna and the helical antenna, so the expense associated with repair is quite high.
Accordingly, while various means have been devised for resolving the problems pertaining to Conventional Art Examples 1-3, these means generally include elements that run counter to a more compact and lightweight radio telephone.
The present invention was developed with the foregoing in view, and is intended to provide an antenna for a radio telephone that, under the limitations imposed by more compact size and lighter weight of a radio telephone, and particularly the condition that the rod-shaped antenna must be of a length readily accommodated within the case, experiences negligible effect by the human body when the rod-shaped antenna extended to the outside of the case for use, and that ensures an electrical length sufficient for emission of radio waves at an angle close to the horizontal direction.
It is a further object of the present invention to provide an antenna for a radio telephone such that, when used with the rod-shaped antenna retracted within the case, the rod-shaped antenna does not produce any unwanted radiation.
It is a still further object of the present invention to eliminate any metal component from the vicinity of the power supply portion of the radio telephone, thereby realizing good impedance characteristics such that free electrostatic capacitance created through the agency of metal components can be ignored in terms of antenna engineering, and this affords improvement in the reduced gain and sensitivity resulting from free electrostatic capacitance.
It is yet a further object of the present invention to provide an antenna for a radio telephone adaptable to greater bandwidth on the part of the service frequency band of a radio telephone.
It is yet a further object of the present invention to provide an antenna for a radio telephone that, in terms of mechanical strength, will not in the course of service, be subject to localized concentrated stresses such that breakage results, and that, in the event that breakage does occur, will entail reduced repair costs.
A first antenna for a radio telephone which pertains to the present invention comprises a helical antenna having an electrical length of xcex/4 (where xcex is the wavelength of the service frequency) and having the basal end thereof, which is arranged on the wall of the case of the radio telephone, connected to the power supply component of the radio telephone, and a rod-shaped antenna extendable and retractable with respect to the case of the radio telephone through the hollow central portion of the helical antenna, with electrical length of about 3xcex/8 and having inductive reactance. This antenna is characterized in that when the rod-shaped antenna is extended from the radio telephone case, the basal end of the rod-shaped antenna and the distal end of the helical antenna are electrically coupled by means of an electrostatic coupling portion having capacitive reactance that cancels out the inductive reactance.
Here, xe2x80x9carranged on the wall of the case of the radio telephonexe2x80x9d is used to mean arranged on the wall of the case such that the rod-shaped antenna is extendable and retractable with respect to the case of the radio telephone through the hollow central portion of the helical antenna. Any location in proximity to the wall is acceptable provided that it is not in proximity to the center of the case interior of the radio telephone, for example, a projecting portion may be provided to the case, and [the antenna] installed on this projecting portion.
In preferred practice, the electrostatic coupling portion of the antenna for the first and second radio telephone will constitute a mechanical structure. Here, xe2x80x9cconstitute a mechanical structurexe2x80x9d is used to mean that, without using any electrical component such as a so-called capacitor, electrostatic coupling is effected, for example, by arranging metal plates in opposition at close proximity, or otherwise effecting electrostatic coupling through arrangement of the basal end of the rod-shaped antenna and the distal end of the helical antenna in close proximity to each other.
In preferred practice, the antenna for a radio telephone which pertains to the present invention will be provided with an insulating grip for extending and retracting the rod-shaped antenna with respect to the case, and an insulating connecting member for connecting the grip and the rod-shaped antenna, these being integrally formed through insert molding at the distal end of the rod-shaped antenna.
Here, xe2x80x9cdistal end of the rod-shaped antennaxe2x80x9d is used to mean that end of the rod-shaped antenna that projects to the outside of the case, and does not mean the end located within the case.
Here, in preferred practice, the length of the connecting member is set to a length such that when the rod-shaped antenna is retracted within the case, the connecting member passes through the helical antenna over the entire length thereof, severing electrical coupling between the helical antenna and the rod-shaped antenna.
In preferred practice, the antenna for the first radio telephone which pertains to the present invention is characterized in that the distance between the power supply portion and the metal retaining member for retaining the rod-shaped antenna is of such a size that the free electrostatic capacitance created between the two is essentially zero.
A second radio device antenna according to the present invention is a radio device antenna of a type that is indifferent to whether or not the rod-shaped antenna freely retracts into or extends from the case of the radio device through the hollow core of a helical antenna. What is characteristic of this second radio device antenna is that it comprises a helical antenna of electrical length xcex/4, deployed in a wall of the case of the radio device, the basal end whereof is connected to the power supply of the radio device, and a rod-shaped antenna having an electrical length of 3xcex/8 and exhibiting inductive reactance, configured such that, when the rod-shaped antenna is being used, the helical antenna and the rod-shaped antenna are electrically coupled in an electrostatic coupling having a capacitive reactance with which the inductive reactance is cancelled out, with a gap between the basal end of the rod-shaped antenna and the distal end of the helical antenna.
In this second radio device antenna, it is preferable that the rod-shaped antenna is capable of being freely raised or lowered, to a deployed position projecting from the radio device case when in use, and to a non-deployed position folded along part of the case when not in use.
By xe2x80x9calong part of the casexe2x80x9d here is meant xe2x80x9calong any one surface of the case of the radio device, external thereto.xe2x80x9d When the rod-shaped antenna is in the non-deployed position, it is preferable that power to that rod-shaped antenna be interrupted. It is not absolutely necessary that the power be cut to this rod-shaped antenna, however, and the rod-shaped antenna and helical antenna may operate together with the rod-shaped antenna folded over along part of the case.
Here, xe2x80x9cfree electrostatic capacitance of essentially zeroxe2x80x9d does not mean that the size of the free electrostatic capacitance is itself essentially zero, but rather that the size of the free electrostatic capacitance is of a size such that it can be ignored in terms of antenna engineering. The xe2x80x9cmetalxe2x80x9d of the metal retaining member does not signify fabrication from metal exclusively, but also includes fabrication from mixtures of metal with non-metals.
In addition, it is preferable that the antenna for the first and second radio telephone which pertains to the present invention is characterized in that the resonance frequency of the rod-shaped antenna and the resonance frequency of the helical antenna are set so as to provide multiple tuning characteristics.
Here, xe2x80x9chaving multiple tuning characteristicsxe2x80x9d is used to means that the resonance frequency of the rod-shaped antenna and the resonance frequency of the helical antenna differ slightly from each other within the service frequency band.
In preferred practice, the antenna for a radio telephone which pertains to the present invention will be provided with a projecting portion situated over a portion of the top face of the case of the radio telephone. The top of the projecting portion will be provided with a retaining portion for retaining the bottom portion of the rod-shaped antenna when the rod-shaped antenna is extended from the case. The helical antenna will be arranged concentrically with respect to the rod-shaped antenna in the space within the projecting portion with a gap between it and the basal end of the rod-shaped antenna.
According to the antenna for the first radio telephone which pertains to the present invention, a helical antenna having electrical length of xcex/4 and a rod-shaped antenna of electrical length of about 3xcex/8 having inductive reactance are constituted such that when the rod-shaped antenna is withdrawn through the hollow interior of the helical antenna and extended from the case of the radio telephone, the basal end of the rod-shaped antenna and the distal end of the helical antenna are electrically coupled by means of an electrostatic coupling portion having capacitive reactance that cancels out the inductive reactance of the rod-shaped antenna. Therefore, when the rod-shaped antenna is extended from the case for use, the rod-shaped antenna of electrical length of about 3xcex/8 and the helical antenna having electrical length of xcex/4 co-operate and function integrally as an antenna similar to a ungrounded type, thereby affording an antenna resistant to the effects of the human body and having good efficiency such that radio wave emission characteristics face slightly downward from the horizontal direction in the vertical plane.
The length of the rod-shaped antenna is such that the electrical length is 3xcex/8, shorter than xcex/2, and thus compared to the antenna of Conventional Art Example 2, the antenna can be accommodated within the case leaving a margin.
Furthermore, with either the first or the second radio device antenna, since the electrostatic coupling portion is formed by a mechanical structure that does not rely on electronic components, the antenna can be produced without the use of electronic components such as capacitors.
By integrally forming a low-mass grip and a connecting member at the distal end of the rod-shaped antenna by means of insert molding, breakage and damage due to flexing of the connecting member are avoided.
By establishing the length of the connecting member connecting the rod-shaped antenna and the helical antenna such that when the rod-shaped antenna is retracted within the case, it passes through the helical antenna over the entire length thereof, severing electrical coupling between the helical antenna and the rod-shaped antenna, power to the rod-shaped antenna can be cut at the instant that the rod-shaped antenna is retracted into the case, thus completely eliminating any unwanted radiation into the interior of the radio telephone.
By setting the distance between the power supply portion and the metal retaining member for retaining the rod-shaped antenna to such a size that the free electrostatic capacitance created between the two is essentially zero, gain and sensitivity can be improved. The impedance characteristics of the antenna are determined by the helical antenna connected to the power supply portion, and since the helical antenna is nominally 50 Ú at electric length of 1/4xcex, there is no need to provide a separate matching circuit, etc., for impedance matching, affording an efficient antenna wherein the antenna itself has matched impedance characteristics.
Still further, by setting the resonance frequency of the rod-shaped antenna and the resonance frequency of the helical antenna slightly different so as to produce multiple tuning characteristics, it is possible to produce a multiple tuning antenna wherein the rod-shaped antenna and the helical antenna resonate simultaneously, as a result affording an antenna with good efficiency having a wide service frequency band, i.e., wide band characteristics.
By providing the antenna for a radio telephone which pertains to the present invention with a projecting portion situated over a portion of the top face of the case of the radio telephone, providing the top of the projecting portion with a retaining portion for retaining the bottom portion of the rod-shaped antenna when the rod-shaped antenna is extended from the case, and arranging the helical antenna concentrically with respect to the rod-shaped antenna in the space within the projecting portion with a gap between it and the basal end of the rod-shaped antenna, there is not a large-mass helical antenna located at the distal end of the rod-shaped antenna, so breakage and damage due to flexure of the rod-shaped antenna do not occur. Since this allows a design wherein concentration of impact load at a single point in the event of being mistakenly dropped onto the floor, etc., with the rod-shaped antenna retracted within the case, snapping, breaking, etc., do not readily occur, affording an antenna experiencing few difficulties and having good maintainability. Even if accidentally dropped, since the helical antenna is situated in the space within the projecting portion, in virtually all cases only the rod-shaped antenna will break, and since it will be sufficient simply to repair the rod-shaped antenna, repair costs can be lowered.
A second radio device antenna according to the present invention is configured such that, when the rod-shaped antenna is in use, the helical antenna of electrical length xcex/4 and the rod-shaped antenna having an electrical length 3xcex/8 and exhibiting reactance are electrically coupled in an electrostatic coupling having a capacitive reactance with which the inductive reactance is cancelled out, with a gap between the basal end of the rod-shaped antenna and the distal end of the helical antenna. Therefore, an efficient antenna can be made with which, as in the first radio device antenna described earlier, when the rod-shaped antenna is in use, the rod-shaped antenna having an electrical length of roughly 3xcex/8 and the helical antenna having an electrical length of xcex/4 will work cooperatively, functioning integrally almost as an ungrounded antenna, not greatly susceptible to influence by the human body, with the electromagnetic radiation characteristics oriented very slightly below the horizontal plane even in the vertical plane. With this second radio device antenna, moreover, if the rod-shaped antenna is made so that it can be freely raised and lowered between a deployed position projecting from the radio device case during use and a non-deployed position folded against a part of the case when not in use, the rod-shaped antenna can be folded over in the non-deployed position when the radio device is being carried about, thus eliminating the obtrusiveness of the rod-shaped antenna, when on the move, and avoiding the danger of the rod-shaped antenna getting bent and thus damaged or destroyed.